systemd-python: correct data_threshold error return value
systemd-python: correct data_threshold error return value

again4you/python-systemd,Awesense/python-systemd,keszybz/python-systemd,Awesense/python-systemd,Awesense/python-systemd,again4you/python-systemd,systemd/python-systemd,keszybz/python-systemd,systemd/python-systemd